In Third Generation Partnership Program (3GPP) Long Term Evolution-Advanced (LTE-A), the introduction of a low powered node such as a pico cell or a femto cell may be exposed to significant uplink interference, since a user equipment or mobile device that may see higher signal to interference noise ratios (SINR) or received signal power from a macro cell may actually have a lower path loss to a lower power base station. Based on the received signal powers, the UE may connect to the macro base station and require a higher transmit power for uplink messages than would be required if communicating with the high powered node. Use of higher uplink power may then generate significant uplink interference for other UEs connected to the low powered node. Uplink interference includes interference from the uplink channels and signals, including the Physical Random Access Channel (PRACH), Physical Uplink Control Channel (PUCCH), Physical Uplink Shared Channel (PUSCH) and Sounding Reference Signal (SRS).
In one embodiment, uplink interference seen at a lower powered node or evolved Node B (eNB) is from interfering UEs served by a macro eNB but geographically located closer to the lower powered node or eNB. The uplink signals from interfering UEs served by the macro eNB may not be time synchronized with the uplink signals from the UEs served by the low powered nodes or eNBs. Hence, a direct use of any interference cancellation schemes in the frequency domain after a receiver fast fourier transform (Rx FFT) is difficult, since after the Rx FFT the interference not only affects the resource blocks allocated to those interfering UEs but also spills over to several neighboring resource blocks which may be allocated to other UEs.
Further, performing interference cancellation in the time domain before the Rx FFT is also difficult, since such interference cancellation needs knowledge of the uplink scheduling information for those interfering UEs at the time of arrival of the uplink signal for proper data demodulation and interference reconciliation.
A UE close to a low power node or eNB but far away from the serving macro eNB still needs to use high uplink transmit power to overcome path loss from the UE to the serving macro eNB. This may not be an efficient use of a UE's limited battery power. In other words, the macro cell coverage defined based on a macro eNB transmitting high power may be suitable for the downlink but not for the uplink, especially from the view point of inefficient use of the UE's limited battery power.
Further, scheduling information exchanged over the X2 interface for interference management may suffer from non-trivial communications delay and the delay varies in different scenarios and could be in the order of tens of milliseconds. This means that the X2 interface cannot be used to exchange downlink or uplink Hybrid Automatic Repeat Request (HARQ) related control information, as the processing times at the eNBs or the UEs for the downlink or uplink HARQ need to be small.